Process control systems, like those used in chemical, petroleum, or other processes, typically include at least one centralized process controller communicatively coupled to at least one host or operator workstation and to one or more field devices via analog and/or digital buses or other communication lines or channels. The field devices, which may be, for example, valves, valve positioners, switches, transmitters (e.g., temperature, pressure and flow rate sensors), etc., perform functions within the process such as opening or closing valves and measuring process parameters.
The process controller receives signals indicative of process measurements made by the field devices and/or other information pertaining to the field devices via an input/output (I/O) device, uses this information to implement a control routine, and then generates control signals which are sent over the buses or other communication channels via the input/output device to the field devices to control the operation of the process. Information from the field devices and the controller is typically made available to one or more applications executed by the operator workstation to enable an operator to perform any desired function with respect to the process, such as viewing the current state of the process, modifying the operation of the process, configuring the process, documenting the process, etc.
In the past, standard communication protocols were developed to enable controllers and field devices from different manufacturers to exchange data using standard formats. One such standard communication protocol, the Modbus protocol, has been in use in process control systems for more than twenty years. The Modbus protocol can define a message structure that controllers will recognize and use regardless of the types of networks over which they communicate. The Modbus protocol establishes a common format for the layout and content of message fields. Modbus defines a process for controllers to request access to other devices, for the controllers and devices to respond to requests, and for error detection and reporting.
During communications on a Modbus network, the protocol determines how each controller or device will know its device address, recognize a message addressed to it, determine the kind of action to be taken, and extract any data or other information contained in the message. If a reply is required, the controller or device will construct the reply message and send it using the Modbus protocol. The Modbus protocol is known in the art and is described in detail in numerous articles, brochures and specifications published, distributed and available from, among others, manufacturers of devices using the Modbus protocol. As a result, the details of the Modbus communication protocol will not be described in detail except to the extent they relate to the disclosed embodiments.
Modbus employs a master-slave query-response cycle for exchanging information between controllers and field devices. Each device can be assigned an address ranging between, for example, 1 and 255. One device (i.e., the “master”) can initiate transactions, such as queries, set point changes, diagnostics, and the like by transmitting a message addressed to another device (i.e., the “slave”). Under the Modbus protocol, the message transmitted by the master device is formatted to include the device address of the slave device, a function code defining the requested action, any data to be sent to the slave device, and error-checking information. The slave device detects and receives the message from the master based on the address in the message, and processes the transaction or function indicated by the function code in the message. After processing the transaction or performing the requested function, the slave transmits a response message constructed using the Modbus protocol and containing information confirming the action taken, any data to be returned to the master, and error-checking information. The Modbus protocol supports both an ASCII mode (wherein messages are transmitted as ASCII characters) and an RTU mode (wherein messages are transmitted as hexadecimal characters).
Data can be exchanged between devices in the Modbus network using register numbers. Each process control parameter used in the Modbus network is assigned a numeric register number within a set range of available register numbers. The devices in the Modbus network are configured to store and exchange the process control parameters according to their assigned register number. In order to obtain the value of a process control parameter from a slave device, a master device formats and transmits a request message including the address of the slave device and the register number for the process control parameter. Upon receiving the request message, the slave device reads the current value for the register number, and formats and transfers a response message with the register number and stored value.
Many legacy industrial Modbus RTU (Remote Terminal Unit) enabled field devices (e.g., actuator, industrial power measuring meter, process measuring transmitter, and many other process measuring devices) remain in operation in industrial plants. Presently and conventionally, to communicate such devices to a control room requires wired connections. This is an extremely expensive and time consuming process. A typical industrial plant may possess thousands of legacy communications devices, leading to lot of wiring going into the control room and in the field from these device. Maintaining these wiring is extremely complicated during maintenance, any breakage in the field wiring takes longer time to fix. Thus, if a solutions can be found to enable such legacy devices to communicate via wireless communications, significant costs savings may accrue.